This is a phase II randomized, multi-institutional cooperative group study designed to evaluate toxicity and efficacy of interleukin-12 given to plateau phase myeloma patients, in order to sustain remission. Because of IL-12's efficacy in causing tumor resistance in mice, and because of the correlation of progression in myeloma with decreased Th1 and cytotoxic T-cell activity, Il-12 is being administered in both arms of this trial to myeloma patients in plateau phase to help sustain remission. Effectiveness of IL-12 to enhance response to pneumococcal and hemophilus influenza B vaccines will be evaluated. Effect of IL-12 in boosting normal immunoglubulins will also be monitored.